The present invention relates to a new and improved apparatus and method for casting single crystal articles, such as airfoils.
It has been suggested that single crystal airfoils may be cast in many different ways, including using a single crystal selector in the manner disclosed in U.S. Pat. No. 3,542,120. Each of the single crystal selectors disclosed in this patent has an upwardly and sidewardly sloping growth passage with a lower end portion exposed to a chill plate and an upper end portion connected with a mold cavity. Exposure of the lower end portion of the growth passage to the chill plate promotes the growth of crystals which are most favorably oriented for growth in an upward direction. This results in the more favorably oriented crystals crowding out the less favorably oriented crystals during solidification of molten metal in the growth passage.
The crystal selector disclosed in the aforementioned U.S. Pat. No. 3,542,120 is used to cast a single crystal article having a cubic unit cell with a vertically oriented axis. The horizontal or &lt;010&gt; axes of the cubic unit cells are randomly oriented. It has been found that it is highly desirable to control the secondary or horizontal orientation of the cubic unit cells of a single crystal article in order to maximize the properties of the article.
It has been suggested that a pair of chill plates could be used to obtain a doubly-oriented single crystal casting, that is a casting having cubic unit cells with both vertical and horizontal axes in a preselected orientation relative to a central axis of the casting. Thus, U.S. Pat. No. 3,572,419 discloses an apparatus for casting doubly-oriented single crystal airfoils by using two separate chill plates, that is, a first chill plate having a horizontal major side surface and a second chill plate having a vertical major side surface.
Although the apparatus disclosed in the aforementioned U.S. Pat. No. 3,572,419 may be suitable for producing doubly-oriented single crystal castings in a laboratory, the apparatus disclosed in this patent has not been used to make doubly-oriented single crystal castings on a commercial basis. It is believed that this is because it is extremely difficult to obtain fluid tight seals between a mold and both a horizontal chill plate and a vertical chill plate. In addition, the use of a pair of chill plates requires a more elaborate furnace set-up and more complicated patterns and molds. The combination of these factors increases the cost of a doubly-oriented single crystal casting over a conventional single crystal casting having only a preselected vertical orientation.